Various forms of electrical continuous flow heaters for domestic appliances such as coffee making machines and the like are known, having a tubular heater and a water flow pipe, together with a temperature limiting means to safeguard the appliance.
German specification DE-Al No. 25 07 479 disclosed such an electrical heater arrangement for domestic coffee making machines, comprising a unit, which is bent into an at least substantially annular configuration, comprising a tubular heater and a water flow pipe. The connecting ends of the arrangement, which project out of the annular configuration, are joined together by way of a sheet metal housing which carries a thermostat. German Utility Model No. 74 12 048 also discloses a continuous-flow or through-flow heater for heating water for a coffee making machine, comprising an arrangement of a tubular heater and a water flow pipe, which is of a generally horseshoe-shaped configuration. In that construction, a sheet metal strip which carries a thermostat is secured to the tubular heater as by welding or soldering. Further structures of the general kind set forth above, including thermostats therein, are described in German Utility Model No. 19 29 117 and in German specification DE-Al No. 28 08 184.
In another form of continuous flow heater for an appliance such as a coffee making machine, as disclosed in German published specification (DE-AS) No. 24 41 467, there is a water chamber which is heated by a tubular heater. In that construction, a temperature fuse of micro type is associated with the water chamber, while a further thermostat device is disposed outside the water chamber in the vicinity of the connecting ends of the tubular heater.
All the coffee making machines of the kind discussed above suffer from the problem set out below, which is referred to in some of the above-mentioned specifications but which hitherto has not been solved in a fully satisfactory manner:
The thermostat associated with the heater for controlling the operation thereof is required to perform at least three completely different functions. First of all, it is required to control the temperature of the water which is in the water flow pipe, to such a level that on the one hand the vapour pressure is sufficient to drive the water up through the riser pipe for it to flow out over the coffee powder to make the coffee, while on the other hand the amount of steam produced by operation of the heater should not be excessive, because that involves a waste of energy while also resulting in a snorting noise as the steam issues from the riser pipe, which is often so severe as to be quite unacceptable. Secondly, when the coffee pot or like beverage container is set down on the machine, the container should be maintained at a temperature of about 80.degree. C. so that even after the full amount of water introduced into the supply tank of the machine has flowed through the heater assembly, the coffee produced from the water can still be kept hot over a period of several hours by virtue of the pot or container resting on the heating assembly of the machine. Thirdly, the current supply to the machine should be cut off in the event of overheating, as may occur for various reasons, for example because the tubular heater burns through or melts. It will be appreciated that overheating of that kind can result in a serious risk of fire, particularly when the appliance has a plastic casing as is nowadays generally the case. Although that cause of damage occurs relatively rarely, being generally of the order of magnitude of one to one million, experience has shown that in the event of a machine such as a coffee making machine suffer from overheating, and catching fire as a result, to such an extent as to be completely burnt out, considerable damage may result, being for example to such an extent as to set fire to the room in which the coffee machine is placed, possibly even being completely burnt out.
Various devices and artifices may be employed, and the heating output of the arrangement may be suitably matched to the thermostat employed, in order thereby to control the situation in such a way that one and the same thermostat can simultaneously perform two of the three functions referred to above. However, as far as the applicant is aware, no arrangement has hitherto succeeded in performing all three functions as referred to above, in a manner which is completely satisfactory under practical circumstances.